1. Field of the Invention
The present invention relates in general to a driving control circuit for a light-emitting device. In particular, the present invention relates to a circuit to control the range of a driving current of a light-emitting device.
2. Description of the Related Art
In the present driving circuit of the light-emitting device, the bias current and the modulated current are controlled separately. The modulated current is provided by a differential output stage. The bias current is provided by a circuit source.
FIG. 1 is a circuit diagram of a conventional driving control circuit for a light-emitting device. In FIG. 1, the conventional driving control circuit drives a laser diode LD100. This circuit includes a pair of input terminals 100 and 101, and a differential output circuit 102 to be applied with a digital data signal to drive the laser diode LD100, a current source circuit 105 for supplying a constant current I101 as a driving current to the laser diode LD100 through the differential output circuit 102, and a current source circuit 106 for supplying a constant current I102 as a dc bias current to the laser diode LD100.
The differential output circuit 102 is formed by n-channel MOS transistors Q101 and Q102 whose sources are coupled together, such as by a source-coupled pair of MOS transistors Q101 and Q102. The digital data signal is applied across the non-inverted input terminal 100 and the inverted input terminal 101. The terminal 100 is connected to a gate of the MOS transistor Q102. The terminal 101 is connected to a gate of the MOS transistor Q101. A drain of the transistor Q101 is connected to one end of a load resistor R101. The other end of the resistor R101 is applied with a power supply voltage VDD. A drain of the transistor Q102, which serves as an output terminal of the conventional driving control circuit, is connected to the cathode of the laser diode LD100. The anode of the laser diode LD100 is applied with the power supply voltage VDD.
The current source circuit 105 is formed by n-channel MOS transistors Q103 and Q104 serving as a current mirror, and a reference current source 103 for supplying a constant reference current Iref1 to the transistor Q104. A drain of the transistor Q103 is connected to the coupled sources of the transistors Q101 and Q102. A source of the transistor Q103 is connected to the ground. A drain and a gate of the transistor Q104 are connected in common to a gate of the transistor Q103. A source of the transistor Q104 are connected to the ground. The commonly connected drain and gate of the transistor Q104 are connected to one end of the reference current source 103. The other end of the reference current source 103 is applied with the power supply voltage VDD.
The current source circuit 106 is formed by n-channel MOS transistors Q105 and Q106 serving as a current mirror, and a reference current source 104 for supplying a constant reference current Iref2 to the transistor Q106. A drain of the transistor Q105 is connected to the drain of the cathode of the laser diode LD100, i.e., the output terminal of the conventional driving control circuit of FIG. 1. A source of the transistor Q105 is connected to the ground. A drain and a gate of the transistor Q106 is connected in common to a gate of the transistor Q105. A source of the transistor Q106 is connected to the ground. The commonly connected drain and gate of the transistor Q106 are connected to one end of the reference current source 104. The other end of the reference current source 104 is applied with the power supply voltage VDD.
The above-described conventional driving control circuit of FIG. 1 has the following problems:
The first problem is that the current consumption of the circuit of FIG. 1 is large, because the dc bias current I102 is always consumed during operation irrespective of the existence and absence of light emission and because the driving current I101.
Second, the work voltage in the output terminal is limited because two transmitters are series in the differential output stage. Therefore, the circuit of FIG. 1 does not work when the output work voltage is lower than VDD. The efficiency of the current is reduced and the application of the circuit becomes more complex.
An object of the present invention is to provide a driving control circuit for a light-emitting device that has better efficiency and is able to increase the work voltage in the output terminal.
Another object of the present invention is to provide a driving control circuit for a light-emitting device that automatically adjusts a gate voltage of the output transistor according to a predetermined bias voltage, and a modulated current to control the gate voltage precisely, thereby enabling a decrease in the power consumption and an increase in the range of the work voltage in the output terminal.
The inventive circuit comprises an adjustable gain circuit, a current output circuit, and a level control circuit. The adjustable gain circuit receives a digital signal and generates an output voltage controlled by a first control signal and a second control signal. The current output circuit comprises a first transistor. A gate of the first transistor is coupled to the adjustable gain circuit. The adjustable gain circuit adjusts a range of the gate voltage to generate the high-speed modulated current. The level control circuit sets the voltages of the first control signal and the second control signal.
The level control circuit comprises a duplicate circuit, a first negative feedback current and a second negative feedback current. The duplicate circuit receives a high level signal and a low level signal and generates a first output current and a second output current. The first negative feedback current sets the first current equal to a first predetermined current and outputs a voltage to set a voltage of the first control signal. The second negative feedback current sets the second current equal to a second predetermined current and outputs a voltage to set a voltage of the second control signal. The high level signal raises the gate voltage of the first transistor above a high level voltage. The low level signal lowers the gate voltage of the first transistor below a low level voltage. The adjustable range of the gate voltage is between the high level voltage and the low level voltage.
Furthermore, the invention also provides another driving control circuit for a light-emitting device to generate a high-speed modulated current to drive the light-emitting device. The circuit comprises an adjustable gain circuit, a current output circuit and a level control circuit. The adjustable gain circuit receives a digital signal and generates an output voltage controlled by a first control signal and a second control signal. The current output circuit comprises a first transistor. A gate of the first transistor is coupled to the adjustable gain circuit. The adjustable gain circuit adjusts a range of the gate voltage to generate the high-speed modulated current. The level control circuit sets the voltages of the first control signal and the second control signal.
The level control circuit comprises a level detect circuit, a first negative feedback current and a second negative feedback current. The level detect circuit is coupled to the adjustable gain circuit and generates a first output current and a second output current. The first negative feedback current sets the first current equal to a first predetermined current and outputs a voltage to set a voltage of the first control signal. The second negative feedback current sets the second current equal to a second predetermined current and outputs a voltage to set a voltage of the second control signal. The adjustable range of the gate voltage is between a high level voltage and a low level voltage.